This invention relates generally to the field of disc drive storage devices, and more particularly, but not by way of limitation, to improving the operational response of a disc drive servo system by compensating for an initial nonzero head velocity at the commencement of a position controlled seek.
Hard disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, the data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs. A read channel and interface circuit are provided to recover previously stored data from the discs to the host computer.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., is used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo data written to the surfaces of the discs during manufacturing. The servo system of a disc drive utilizes the servo data in the performance of two primary operations: seeking and track following.
Track following entails the continued positioning of a selected head over a corresponding, selected track. A position-control approach is employed wherein the relative position of the head with respect to the center of the track is determined and compared to a desired position for the head. The resulting position error is used to control the amount of current that is applied to the actuator coil in order to maintain the head at the desired position relative to the track.
Seeking entails the movement of a selected head from an initial track to a destination track. For seeks above a certain length, a velocity-control approach is employed wherein the velocity of the head is repetitively determined and compared to a velocity profile which defines an optimum velocity trajectory for the head as it moves to the target track. The amount of current applied to an actuator coil varies in proportion to the velocity error, the actuator coil being part of a voice coil motor used to control the position of the head.
For shorter seeks (such as about 100 tracks or so, depending on the configuration of the drive), some disc drives use a position control approach wherein reference current, velocity and position profiles are generated and used to advance the head to the destination track. Sometimes referred to as xe2x80x9cmodel referencexe2x80x9d seeks, such seeks can provide faster response and better settling characteristics than velocity-controlled seeks. A model reference seek typically involves selecting a sinusoidally based reference current profile (such as a 1-cos function), which is scaled using an appropriate scale factor based on the number of tracks in each seek. The scaled current profile is then integrated to provide the corresponding reference velocity and reference position. Model reference seeks are discussed, for example, in U.S. Pat. No. 6,031,684 issued to Gregg.
While advantageously enabling servo systems to provide enhanced positional control, increases in track density make model reference seeks harder to perform. Smaller physical movements of the head from one track to the next require less current, which increases variation in head position, since the servo system has less control over the actuator. Moreover, as track densities increase, errors in the radial position of the servo data as well as vibrations increasingly tend to cause the head to have a small, but nonzero, velocity (either toward the inner diameter or the outer diameter of the disc). The servo system continually operates to compensate for such errors by adjusting the position of the head to follow the selected track.
It has been found that a nonzero initial velocity of the head can significantly degrade the ability of a servo system to accurately move the head from the initial track to the destination track. If the head is already moving toward the destination track at the commencement of a model reference seek, the reference control inputs can cause the head to undershoot the destination track. Conversely, if the head is moving away from the destination track when a model reference seek is initiated, the reference input control may cause the head to overshoot the destination track. In both cases, additional time is required to complete the movement of the head to the desired position over the destination track, reducing transfer rate performance of the disc drive.
Accordingly, with continued advancements in data storage capacities, there is a need to enable drives to perform short, position controlled seeks that compensate for initial nonzero head velocities. It is to such improvements that the present invention is directed.
The present invention provides a method and apparatus for optimizing the gain of a disc drive servo circuit used to position a head adjacent tracks on a disc of the disc drive. In a preferred embodiment, the present invention provides a method and apparatus for compensating the feed-forward control employed by the model reference controller during a position controlled seek.
The model reference controller is a control system that applies a feed-forward current to the actuator to perform seeks. Predetermined feed-forward tables are used to generate the feed-forward currents based upon the length of the required seeks. The model reference controller uses a plant model and observer circuitry to generate a reference position and reference velocity from the feed-forward current, upon which the loop is closed. The multi-rate observer circuitry estimates the position of the head for each of a plurality of multi-rate periods of the seek, with the position error corresponding to the difference between the reference position and the estimated position at each period.
A current profile is scaled by first and second gains, with the first gain being applied during the acceleration portion of the seek and the second gain being applied during the deceleration portion of the seek. The first and second gains are selected to have different magnitudes when the initial velocity of the head is nonzero.
In addition to providing a model reference controller which accounts for initial head velocity, the present invention effectively reduces the time spent tuning the model reference seek controller loop. The present invention allows the model reference controller to xe2x80x9cself-tunexe2x80x9d according to pre-calculated factors corresponding to each feed-forward reference table.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.